Method of registrably aligning fabricated wafers preceding bonding

ABSTRACT

A method of registrably aligning wafers prior to bonding uses an assembly jig having a plurality of upstanding aligning elements each of which is arranged to registrably align with one of a predetermined number of locating features of the wafers. The locating features of the wafers are arranged about a corresponding upstanding aligning element of the jig prior to bonding the wafers.

FIELD OF THE INVENTION

The invention relates generally to the field of wafer bonding. More particularly the invention concerns a method of precisely aligning wafers prior to bonding.

BACKGROUND OF THE INVENTION

Single wafer integration, wherein all the components of a device are formed simultaneously on one wafer, has been a standard and successful practice in the semiconductor industry for decades. In the emerging fields of micromechanics and microsystems, however, advanced designs increasingly require a multiple wafer integration strategy where the various components of a device are fabricated onto a plurality of wafers and then the processed wafers are bonded together to form the final product. The design situations that necessitate multiple wafer integration include complicated three-dimensional geometries, incompatibilities among fabrication processes and, particularly, the need to build device components on a wide palette of non-silicon starting wafer material types.

A number of bonding techniques are known that can produce strong, reliable bonds between wafers. Fusion bonding is a direct bonding process where two clean, flat surfaces, such as silicon, silicon dioxide, or silicon nitride, are covalently bonded through the application of pressure and heat. In anodic bonding a silicon surface and a borosilicate glass surface are fused through the application of strong electric fields and heat. Adhesive bonding is applicable to the widest range of wafer materials, but the bond strengths achieved are typically lower than those for either fusion or anodic bonding. Independent of the bonding method used, the first step in wafer bonding is to position the wafers in fixed relation.

There are applications where wafer bonding is performed without a precise alignment of the wafers to be bonded. If at least one of the wafers contains no device features then only a very coarse alignment may be necessary. This is the case for high purity silicon on insulator (SOI), where a bare silicon wafer is fusion bonded to a silicon dioxide-coated silicon wafer, and also when a bare wafer is bonded to a device wafer to serve as a cap or seal. In general, however, wafer bonding requires the initial steps of accurately aligning the components of a first wafer with the components of a second wafer and then holding the wafers in fixed relation for the bonding process.

Current methods for aligning wafers prior to bonding are time-consuming and require expensive equipment. In U.S. Pat. No. 5,236,118, titled, "Aligned Wafer Bonding" by Bower et al. described is a wafer bonding process using a wafer aligner with precision mechanical stages and a sophisticated imaging system to optically align the wafers. In particular, Bower et al. teaches the use of infrared viewing to facilitate alignment of wafers. Wafer aligners based on infrared or alternative optical techniques are offered commercially by several semiconductor equipment manufacturers. They compare in complexity and price to lithographic contact aligners and require a high level of skill to operate. For high volume manufacturing of wafer bonded devices, it would be advantageous to have a wafer bonding process with a low-cost wafer alignment step that did not require expensive capital equipment and could be performed quickly by unskilled operators or robotic assemblers. The use of commercial wafer aligners is currently restricted to the alignment of two wafers at one time. It would be a further advantage then to have a wafer alignment process that, in addition to the aforementioned benefits, could align three or more wafers for simultaneous bonding. Therefore, there persists a need in the art for a method of aligning wafers prior to bonding that is simple and inexpensive to construct.

SUMMARY OF THE INVENTION

It is, therefore, an object of the invention, to provide a mechanical method for precisely aligning wafers prior to bonding without the need for precision movements or optical alignment steps.

Another object of the invention is to provide a method for precisely aligning wafers prior to bonding where the wafers contain features or materials that are incompatible with optical aligning techniques.

Yet another object of the invention is to provide a method suitable for the precise alignment of three or more wafers for simultaneous bonding.

To accomplish these and other objects of the invention, there is provided a method for registrably aligning wafers prior to bonding, comprising the steps of:

providing an assembly jig consisting of a substantially planar platform having a plurality of upstanding aligning elements spatially arranged about one face of the platform;

providing a first wafer for bonding, said first wafer having a first predetermined pattern formed therein and a predetermined number of first locating features corresponding to the plurality of upstanding aligning elements on said platform, wherein each one of the first locating features has a predetermined spatial relationship with the first predetermined pattern;

providing at least a second wafer for bonding to said first wafer, said second wafer having a second predetermined pattern formed therein and a predetermined number of second locating features corresponding to the plurality of upstanding aligning elements on said platform, wherein each one of the second locating features has a predetermined spatial relationship with the second predetermined pattern;

registrably aligning each one of the predetermined number of first locating features of said first wafer with a corresponding one of said plurality of upstanding aligning elements on said platform;

arranging each one of the predetermined number of locating features of said first wafer about the correspondingly aligned one of said plurality of upstanding aligning elements on said platform;

registrably aligning each one of the predetermined number of second locating features of said second wafer with a corresponding one of said plurality of upstanding aligning elements on said platform; and

arranging each one of the predetermined number of second locating features of said second wafer about the correspondingly aligned one of said plurality of upstanding aligning elements on said platform such that a face of the first wafer is in intimate contact with a face of the second wafer for bonding.

The method of the present invention has numerous advantageous effects over existing developments including: low cost; and ease and speed of manufacture. Moreover, a further advantage of the method of the invention is that it enables the alignment of multiple wafers for simultaneous bonding.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing as well as other objects, features and advantages of this invention will become more apparent from the appended Figures, wherein like reference numerals denote like elements, and wherein:

FIG. 1 is an exploded view of an apparatus for aligning wafers for bonding;

FIG. 2 is an oblique view of the wafers positioned on an aligning platform;

FIG. 3 is a partial cross-sectional view taken along line III--III of FIG. 1 showing a preferred construction method;

FIG. 4 is an oblique view of a wafer showing specific features important to the present invention;

FIGS. 5A-5E are fragmentary sections of various possible sidewall profiles of locating features;

FIG. 6 is a process diagram showing alignment relationships;

FIG. 7 is an oblique view of the wafers positioned on an assembly jig which contains an ultrasonic transducer and a heater; and

FIGS. 8A-8D are top views of various configurations of aligning elements and locating features.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings and particularly to FIGS. 1 and 2, the method for registrably aligning fabricated wafers prior to bonding, broadly defined, includes the steps of providing an assembly jig 10 consisting of a substantially planar platform 12 having a plurality of upstanding aligning elements 14 spatially arranged about one face 16 of the platform. FIGS. 1 and 2 illustrate one preferred embodiment wherein three upstanding aligning elements 14 are employed.

According to FIG. 1, first wafer 18 suitable for bonding, has a first predetermined pattern 20 formed therein and a predetermined number of first locating features 22, 24, and 26 corresponding to the three upstanding aligning elements 14 on said platform 12. It is important to our invention that each one of the first locating features 22, 24, and 26 has a predetermined spatial relationship with the first predetermined pattern 20.

Referring again to FIG. 1, a second wafer 28 is provided for bonding to the first wafer 18. Second wafer 28 has a second predetermined pattern 30 formed therein and a predetermined number of second locating features 32, 34, and 36 also corresponding to the three upstanding aligning elements 14 on the platform 12. Similar to the first locating features 22, 24, and 26, each one of the second locating features 32, 34, and 36 has a predetermined spatial relationship with the second predetermined pattern 30.

Referring to FIG. 2, once the wafers 18, 28 for bonding are provided, they must then be registrably aligned with the assembly jig 10. Thus, each one of the predetermined number of first locating features 22, 24, and 26 of the first wafer 18 is registrably aligned with a corresponding one of the upstanding aligning elements 14 on platform 12.

According to the method of the invention, each one of the predetermined number of first locating features 22, 24, and 26 of the first wafer 18 is arranged about a correspondingly aligned upstanding aligning elements 14 on platform 12.

Referring once again to FIG. 1, in a similar manner, each one of the predetermined number of second locating features 32, 34, and 36 of the second wafer 28 is registrably aligned with a corresponding upstanding aligning elements 14 on platform 12.

Still referring to FIG. 1, finally, each one of the predetermined number of second locating features 32, 34, and 36 of second wafer 28 is arranged about a correspondingly aligned upstanding aligning elements 14 on platform 12 such that a face 40 of the first wafer is in intimate contact with a face 42 of the second wafer in preparation for bonding. FIG. 2 shows the first wafer 18 and the second wafer 28 registrably aligned to each other with their respective faces 40 and 42 (not visible) in intimate contact and ready for bonding.

The preferred alignment mechanism of the present invention is kinematic alignment of the wafers 18, 28. According to kinematic design principles, an object can be precisely and repeatably aligned by establishing point contact at the minimum number of points required to restrain the object in a desired position and orientation. Fixing an object in space requires six points of contact, but since the platform face 16 constrains the wafers 18, 28 in a plane, only three additional points of contact are needed for accurate alignment. Further preferred embodiments of the invention are easily understood in view of the kinematic design principle.

Referring to FIGS. 1 and 3, proper construction of assembly jig 10 is important to the present invention. Flatness of the platform face 16 can be assured by known precision maching methods such as jig gringing, cylindrical grinding, and jig boring, for example. We prefer using jig grinding. It is also crucial that the upstanding aligning elements 14 be perpendicular to the platform face 16. Deviations from perpendicularity may cause alignment error between wafers 18, 28. In FIG. 3, the preferred construction method for the upstanding aligning elements 14 is depicted as taken along section line III--III of FIG. 1. Upstanding aligning element 14 is seated into a machined hole (not explicitly shown) in platform 12. A counter bore pocket 15 provides clearance for a jig grinding tool 17 which provides a precise perpendicular finish to the outer surface of the upstanding aligning element 14. It is preferable that all the upstanding aligning elements 14 be jig ground in a single set-up operation. To aid in the aligning process, a taper 13 is formed in the upstanding aligning element 14 either before or after insertion into platform 12.

Assembly jig 10 is preferably constructed of materials selected from the group consisting of aluminum, stainless steel, tool steel, ferrous alloys, nickel, nickel alloys, molybdenum, tungsten, quartz, aluminum oxide, tungsten carbide, ceramics, and low thermal coefficient of expansion alloys such as invar. Selection of the preferred material for fabricating assembly jig 10 is governed by intended application of the bonding method expected to be used. For instance, if the assembly jig 10 includes heating means (not shown) to facilitate the bonding process, then materials with high thermal conductivity should be used, such as nickel.

Referring now to FIG. 4, a first wafer 18 is provided with specific features and aspects important to the invention. First wafer 18 comprises first wafer face 40 and an opposite first wafer face 41. A first predetermined pattern 20 of device components is formed on first wafer face 40 of first wafer 18.

In wafer-based fabrication, device components are generally formed through a series of individual process steps including lithographic definition, material removal (e.g. plasma etching), and material deposition (e.g. sputtering and physical vapor deposition). The size, shape, and position of component features is determined by the lithographic definition steps.

To facilitate the alignment of successive lithographic definition steps a reference coordinate system, shown as arrows X₁ -Y₁, in FIG. 4, is established by forming alignment marks 50 on wafer 18. Using known lithographic art, the device components contained in the first predetermined pattern 20 can be formed with a positional accuracy of the order of 0.1 micrometers (μm) or better relative to alignment marks 50 and the coordinate system X₁ -Y₁.

Referring again to FIG. 4, a set of first locating features 22, 24, and 26 are provided preferably at the periphery of wafer 18. The first locating features 22, 24, and 26 are openings that extend completely through wafer 18. There are a number of known techniques suitable for the production of locating features 22, 24, 26. In silicon wafers it is preferable to plasma etch first locating features 22, 24, and 26. The plasma etching technique known as the "Bosch Process" and described in German Pat. No. DE 43 17 623 A 1 has been commercialized by several semiconductor equipment manufacturers. The Bosch Process provides fast, anisotropic etching of silicon and is a particularly suitable fabrication method. Photoetchable glass such as Foturan™ made by Schott Corporation is suitable for forming first locating features 22, 24, and 26 in glass wafers. Other well known methods including molding, electroforming, drilling, laser ablation, and electro-discharge machining may be suitable to form locating features in wafers of other materials.

According to FIG. 4, the set of first locating features 22, 24, and 26 establish a second coordinate system as indicated by arrows A₁ -B₁. First locating features 22, 24, and 26 are formed through a series of individual process steps including lithography and etching. The size, shape, and position of the first locating features 22, 24, and 26 are determined by the lithographic definition steps which are referenced to the alignment marks 50 and the coordinate system X₁ -Y₁.

Referring now to FIGS. 5A-5E, fragmentary sections of sidewall profile 62 of the first locating features 22, 24, and 26 of first wafer 18 are depicted sidewall profile 62 is governed by an etch mask 60 and specific etching conditions. In FIGS. 5A-5E, reference line A--A designates the vertical plane defined by the end edge of etch mask 60 which establishes the coordinate system A₁ -B₁, as shown in FIG. 4. Using known lithographic art, etch mask 60 can be formed with a positional accuracy of the order of 0.1 μm or better relative to alignment marks 50 and the coordinate system X₁ -Y₁. Thus, the root mean square alignment accuracy of the first predetermined pattern 20 to the first locating features 22, 24, and 26 will be on the order of 0.14 μm or better, assuming that etching of the locating features 22, 24, and 26 can be performed with high fidelity to the etch mask 60. FIGS. 5A-5E schematically show a variety of sidewall profiles 62 that may be produced in practice.

According to FIG. 5A, a perfectly vertical sidewall profile 62 is depicted with no undercutting of the etch mask 60. This is one preferred embodiment of sidewal profile 62. In FIG. 5B, the etch conditions have produced a vertical sidewall profile 62 but with an undercut 64 of the etch mask 60. If the amount of undercut 64 is reproducible on all the wafers to be bonded then this profile can give satisfactory aligning results. According to FIG. 5C, the etching conditions have produced a sidewall profile 62 that slopes away from the etch mask 60. Such a profile is called "overcut." If strict control can be kept over the amount of overcut and the thickness of the wafers to be aligned, then the profile of FIG. 5C may give acceptable alignment results; however, the condition of FIG. 5C is to be generally avoided.

Referring to FIG. 5D, an unacceptable sidewall profile 62 is illustrated where fluctuations in the etch process have created an unpredictable modulation. Recalling that the kinematic principle specifies point contact to positionally determine an object, we note that in practice true point contact can never be achieved between two objects because of material deformation. In FIG. 5E, another preferred embodiment is illustrated where the sidewall profile 62 is initially vertical and then tapers away in the so-called "undercut" profile. The sidewall profile 62 of FIG. 5E provides a predictable contact site along the vertical plane A--A which facilitates accurate alignment with upstanding element 14. This configuration of sidewall profile 62, moreover, assures that the contact site will be closely aligned with end edge A--A of the etch mask 60.

Referring to FIGS. 1 and 2, a second wafer 28 is provided for aligning and bonding to first wafer 18. The details of construction of second wafer 28 are similar to those of first wafer 18, including the establishment of two coordinate systems based on alignment marks and locating features. A distinction between wafers 18, 28 is that second locating features 32, 34, and 36 on second wafer 28 may be mirrored in orientation relative to first locating features 22, 24, and 26 on first wafer 18. This is done in order to obtain the desired contact of first wafer face 40 to second wafer face 42, as shown in FIGS. 1 and 2.

FIG. 6 illustrates the alignment architecture involved in aligning two wafers 18, 28 according to the present invention. First and second wafers 18, 28 are provided with first and second predetermined patterns of device components 20, 30, respectively, arranged with respect to coordinate systems X₁ -Y₁, and X₂ -Y₂, respectively. The goal of aligning coordinate systems X₁ -Y₁, and X₂ -Y₂, is achieved by creating auxiliary coordinate systems, A₁ -B₁, and A₂ -B₂, based on locating features, which can be simply and accurately aligned using the method of the present invention. As described above, the alignment accuracy of A₁ -B₁, with respect to X₁ -Y₁ and, similarly, A₂ -B₂ with respect to X₂ -Y₂ is determined by photolithography and etch processes and can be held to the order of 0.14 μm or better. The jig assembly 10 of the present invention should be capable of aligning coordinate system A₁ -B₁ to A₂ -B₂ with accuracy of the order of 1.0 μm or better. Thus, the predetermined patterns of device components 20, 30 on the two wafers 18, 28 can be aligned with accuracy of the order of 1.0 μm or better without the use of expensive mechanical stages or complex optical alignment systems.

Referring now to FIG. 7, the assembly jig 10 is equipped with a vibration transducer 80 to apply vibrational energy to the first wafer 18 and second wafer 28 to assist in arranging the wafers onto upstanding aligning elements 14. The vibrational energy is preferably in the frequency range of 1 KHz to 100 MHz. To aid thermally based bonding techniques, the assembly jig 10 may be equipped with a heater 70 and temperature controller 75.

Referring again to FIG. 4, the geometry of the locating features 22, 24, and 26 is of particular importance to the present invention. In one preferred embodiment locating feature 22 has a substantially V-shape through-opening in wafer 18 for providing two points of contact with an upstanding aligning element 14.

In another preferred embodiment, locating feature 24 is a through opening in wafer 18 with at least one substantially straight edge for providing a single point of contact with an upstanding aligning element 14. In yet another preferred embodiment locating feature 26 comprises a biasing means for mechanically preloading the wafer 18 on assembly jig 10. One preferred biasing means is a cantilever-shaped spring 21 as shown in FIG. 4 (also shown in FIGS. 1 and 2). Other spring shapes employing multiple cantilevers, serpentines, and other geometries are well known in the art and may be employed in the present invention. In general, the spring shape, taking into account the mechanical stiffness of the wafer material, will be optimized for best alignment performance on a case-by-case basis.

Referring to FIGS. 8A-8D, top views of various configurations of upstanding aligning elements 14 and locating features 22, 24, 26, 52, 54, 57 are illustrated. FIG. 8A shows a preferred embodiment wherein only two upstanding aligning elements 14 are required. Locating feature 22 provides two points of contact between the wafer 18 and the upstanding aligning element 14. Locating feature 57 includes a contoured portion 58 and a biasing member 59, wherein the contoured portion 58 defines a rotational alignment of wafer 18 and the biasing member 59 preloads wafer 18 against the upstanding aligning element 14. According to FIG. 8B, an alternative embodiment is depicted in which four upstanding aligning elements 14 are used. Three locating features 24 each provide a single point of contact for kinematic alignment while locating feature 26 provides to preloading. Referring to FIGS. 8C and 8D, preferred embodiments, are illustrated wherein locating features are moved further to the wafer periphery in order to maximize the area available for device component fabrication.

In FIG. 8C, the wafer is provided with a substantially V-shaped notched locating feature 52 wherein the V-shaped surfaces form part of the outer edge of wafer 18. The V-shaped notched locating feature 52 provides two points of contact between the wafer and the upstanding aligning element 14. The third point of contact is provided by a substantially straight-edged notched locating feature 54, wherein the straight-edged surface forms part of the outer edge of wafer 18. In FIG. 8C locating feature 26 provides preloading.

FIG. 8D illustrates an alternative embodiment wherein preloading is provided by an external preloading means 56. The external preloading means 56 is preferably a spring affixed to platform 12. If platform 12 is oriented such that the platform face 16 is substantially vertical, then the external preloading means 56 can be an external weight or the weight itself of wafer 18.

As illustrated in FIGS. 1 and 2, it will be appreciated that the wafers 18, 28 to be aligned may have device features on one face only, Alternatively, wafers 18, 28 may have device features on both opposite faces or there may be no device features on either face. Also, it will be recognized that the present invention is well adapted to aligning three or more wafers simultaneously by repeating the aligning sequence shown in FIGS. 1 and 2.

The invention has been described with reference to preferred embodiments. However, it will be appreciated that variations and modifications can be effected by a person of ordinary skill in the art without departing from the scope of the invention.

PARTS LIST

10 assembly jig

12 platform

13 taper

14 upstanding aligning elements

15 counter bore pocket

16 platform face

17 jig grinding tool

18 first wafer

20 first predetermined pattern

21 spring

22 first locating feature

24 another first locating feature

26 yet another first locating feature

28 second wafer

30 second predetermined pattern

32 second locating feature

34 another second locating feature

36 yet another second locating feature

40 first wafer face

41 opposite first wafer face

42 second wafer face

50 alignment marks

52 V-shaped notched locating feature

54 straight-edged notched locating feature

56 external preloading means

57 locating feature

58 contoured portion

59 biasing member

60 etch mask

62 sidewall profile

64 undercut

70 heater

75 temperature controller

80 vibration transducer 

What is claimed is:
 1. A method for registrably aligning wafers prior to bonding, comprising the steps of:providing an assembly jig consisting of a substantially planar platform having a plurality of upstanding aligning elements spatially arranged about one face of said platform; providing a first wafer for bonding, said first wafer having a first predetermined pattern formed therein and a predetermined number of first locating features corresponding to said plurality of upstanding aligning elements on said platform, wherein each one of said first locating features has a predetermined spatial relationship with said first predetermined pattern; providing at least a second wafer for bonding to said first wafer, said second wafer having a second predetermined pattern formed therein and a predetermined number of second locating features corresponding to said plurality of upstanding aligning elements on said platform, wherein each one of said second locating features has a predetermined spatial relationship with said second predetermined pattern; registrably aligning each one of said predetermined number of first locating features of said first wafer with a corresponding one of said plurality of upstanding aligning elements on said platform; arranging each one of said predetermined number of locating features of said first wafer about the correspondingly aligned one of said plurality of upstanding aligning elements on said platform; registrably aligning each one of said predetermined number of second locating features of said second wafer with a corresponding one of said plurality of upstanding aligning elements on said platform; and arranging each one of said predetermined number of second locating features of said second wafer about the correspondingly aligned one of said plurality of upstanding aligning elements on said platform such that a face of the first wafer is in intimate contact with a face of the second wafer for bonding.
 2. The method recited in claim 1 wherein said assembly jig is provided with two upstanding aligning elements and wherein said first wafer is provided with a first locating feature etched therein having a through opening configured in a substantially V-shape for providing two points of contacts with one of said two upstanding aligning elements, and a second locating feature etched therein having a contoured portion and a biasing member, wherein said contoured portion defines a rotational alignment of said first wafer and said biasing member preloads said first wafer against said upstanding aligning elements.
 3. The method recited in claim 1 wherein said assembly jig is provided with three upstanding aligning elements and wherein said first wafer is provided with a first locating feature etched therein having a through opening configured in a substantially V-shape for providing two points of contacts with one of said two upstanding aligning elements, and a second locating feature etched therein having a substantially straight edge portion for providing one point of contact with another of said upstanding aligning elements and a third locating feature for providing preloading of said first wafer against said upstanding aligning elements.
 4. The method recited in claim 1 wherein said assembly jig is provided with three upstanding aligning elements and wherein said first wafer is provided with a substantially V-shaped notched first locating feature etched therein for providing two points of contacts with one of said two upstanding aligning elements, said V-shaped notched locating feature forming part of the outer edge of the wafer, and a straight edged notched second locating feature etched therein having a substantially straight edge portion for providing one point of contact with another of said upstanding aligning elements, said straight edge notched second locating feature forming part of the outer edge of said wafer.
 5. The method recited in claim 1 wherein after the step of arranging each one of the predetermined number of second locating features further including a step of imparting vibrational energy to said assembly jig to facilitate alignment of said first wafer to said at least second wafer.
 6. The method recited in claim 5 wherein after the step of imparting vibrational energy to said assembly jig providing thermal energy to said aligned wafers for promoting bonding therebetween. 